Availability of clean water is a growing world-wide challenge. Consequently, development of efficient water purification, desalination and recycling technologies is an important topic on the world-wide research agenda.
Membrane filtration is considered a very powerful purification technology to tackle this problem. The majority of the membranes used for water filtration have long been polymeric membranes. However, more recently also ceramic membranes are finding their way into this field. The main benefits of ceramic membranes are their high chemical and thermal stability enabling chemical and/or thermal regeneration and sterilization by aggressive chemicals and/or hot steam. Moreover, their high mechanical stability enables high pressure back-flushing. As a consequence, despite their higher cost, ceramic membranes are becoming an economically feasible alternative for polymeric membranes in water treatment.
A critical issue in the development of effective membrane processes (both for polymeric and ceramic membranes) is the decline in system performance due to membrane fouling. This limits the economic efficiency of the operation and slows down large scale industrial applications of membranes especially in case of fouling caused by the adsorption of dissolved matter onto the membrane surface and/or into the membrane pores. This type of fouling is considered irreversible fouling and can generally only be removed by chemical cleaning.
Membrane fouling can be decreased by optimization of feed pre-treatment (e.g. via ultrafiltration, microfiltration, flocculation, ozonation and/or UV oxidation), and regular physical and chemical cleaning. Additional measures involve a careful selection of membrane, module design, and operating parameters.
A more sustainable approach is the prevention of the undesired adsorption processes by membrane-surface modification. Although poorly understood, it is generally accepted that fouling of polymeric membranes in water treatment decreases with an increase in hydrophilicity of the membrane material. Consequently, research has been performed to increase polymer membrane hydrophilicity by grafting, plasma or other surface treatment.
Ceramic membranes, and particularly silicon and/or metal oxide and hydroxide membranes, generally are intrinsically hydrophilic and consistently show relative low fouling in water treatment. Nevertheless, also these membranes become less effective over time due to fouling.
Grafting of ceramic materials with phosphonic acids is known to result in stable modified metal oxide surfaces (Mutin et al.; J. Mater. Chem. 2005, 15, 3761). International patent application WO 2010/106167 describes another stable grafting of organic functional moieties onto the surface of ceramic membranes in order to increase the membrane hydrophobicity or change its affinity.